It is known in the art to determine the position of a system using received signals, e.g. a global positioning system (GPS) satellite or pseudolite system transmits a signal to a GPS receiver to enable the receiver to determine its position. The GPS receiver, having an unobstructed view of the GPS satellite, receives the satellite-transmitted signal which includes data values indicating the transmitting satellite's location and the transmission time. The GPS satellites transmit the signal synchronously and because the GPS receiver distance from the GPS satellites varies, the transmitted signals are received at different times. The GPS receiver estimates the distance to each satellite by estimating the amount of time required for the signal to reach the receiver from the satellite.
Disadvantageously, in order to determine position, the GPS receiver must spend a significant amount of time to collect sufficient satellite data. At the same time, the GPS receiver decoding the satellite data requires a stronger GPS satellite signal than is required for position determination when satellite and time information are already known. Assisted-GPS (A-GPS) as a concept has gained significant popularity recently in light of stringent time to first fix (TTFF), i.e., first position determination, and sensitivity requirements of United States Federal Communications Commission (FCC) E-911 regulations. The E-911 regulations require wireless devices being used for emergency calls to report, among other things, the telephone number, location of the tower receiving the emergency call, and location information, e.g. latitude and longitude of the calling device to within 50 to 100 meters, of the wireless device. Problematically, wireless devices may be used in locations where there is no unobstructed satellite view, e.g. inside buildings or under tree cover, and consequently providing accurate location information responsive to the E-911 regulations is impossible.
A typical A-GPS service using a wireless link provides a GPS receiver with an information signal for determining the GPS receiver's approximate position, time synchronization mark, satellite ephemerides, and satellite Dopplers. Different A-GPS services may omit some of these parameters. The A-GPS service provided information allows a GPS receiver to perform a “hot” fix without requiring time to be spent on searching for the satellite Doppler and collecting ephemeris information.
Another important use of the assistance signal, i.e. the signal providing the above-described additional information, is to provide the GPS information bits in real-time to a GPS receiver. By receiving the information bits over a wireless link, a GPS receiver can perform longer than 20 millisecond (ms) coherent accumulation of the GPS signal and thereby increase the receiver sensitivity and allow performance of “in-door”, i.e. obstructed, position determination satisfying the E-911 regulations.
Limitation of Current Techniques
Existing A-GPS approaches known to the inventor have the following major drawbacks:                A. Requirement for point-to-point communication between a GPS receiver and a base station;        B. Time delay between the moment when the GPS information bits are received by a GPS receiver via GPS satellite-transmitted signal and the moment when they become available via an A-GPS transmission;        C. Point-to-point receiver-base communication is required every time a GPS receiver requires information bit values because only the base station is capable of removing the information bits from a low signal-to-noise ratio (SNR) signal; and        D. Wireless communication between a GPS receiver and a base station is always required even if the GPS receiver had an unobstructed view of the sky for a short time prior to the moment when a high sensitivity fix is required, i.e., there is no A-GPS memory.        
Each of the above-mentioned drawbacks is now described in detail.
A. Point-to-Point (P-P) Communication Constraints
The state of the art A-GPS message assumes a bi-directional, i.e. two way, point-to-point wireless communication link between a GPS receiver and a base station which collects and transmits the A-GPS information. In particular, a GPS receiver initiates the communication by requesting A-GPS information from the base station. Responsive to the GPS receiver request, the base station transmits the requested A-GPS message tailored for the particular receiver on a point-to-point communication link. The fact that the link is often realized via a wireless phone network or a packet switching service does not change the fact that the communication exchange is performed on a point-to-point basis.
A point-to-point communication protocol puts significant constraints on the overall system:                1. A mobile GPS device or a GPS receiver must have transmit capability in addition to receive capability. This increases cost, complexity, and power use of the mobile GPS device, as well as, limiting the range of the A-GPS service because a stationary base station typically has a much more powerful transmitter than the mobile GPS device.        2. As the number of mobile GPS devices served by a particular base station increases, the overall system communication bandwidth requirements increase.B. Time Delay Constraints        
The real-time requirements for transmission of the A-GPS information bits is even more restrictive than the above P-P communication constraint. Information bit sequence message needs to be individually constructed for each receiver.
There is a time delay between the information bits received by a GPS receiver and the time the same bits are received via an A-GPS message transmitted from a base station to the GPS receiver. Therefore, a receiver must use the information from the A-GPS message in post processing, thereby requiring more complicated algorithms and, most importantly, preventing use of the A-GPS message information bits for improving the sensitivity of the phase and frequency real-time tracking loops. Therefore, the current structure of the A-GPS message, i.e. the bits of information, is used only for enhancement of the GPS signal detection and is not suitable for improving tracking ability.
C. Tracking Constraints
Pseudo tracking can be achieved even without performing explicit GPS signal phase tracking inside a GPS receiver. A GPS receiver can record, approximately 500 ms of data, then use another 500 ms to request data bits for the sequence from the base station and use the received information to produce an individual high sensitivity fix. From a user perspective, such a GPS receiver produces a fix every second giving the appearance of tracking; however, the algorithm is less accurate than using real-time GPS signals, satellite signal-based tracking and requires constant point-to-point communication between the GPS receiver and base station.
D. “No A-GPS Memory” Constraint
Current A-GPS systems require a bi-directional, i.e. two way, point-to-point wireless communication every time a new high sensitivity fix is required. Assuming a GPS receiver is required to wake up every 30 minutes, determine its position, and if the position has changed from the last fix by more than a predetermined distance, then an alarm must be transmitted to the base station. Current A-GPS systems require a new point-to-point communication every time an assisted fix is desired. The fact that the assistance (A-GPS signal) was transmitted some time ago for the previous fix is not used.